A DRAM (Dynamic Random Access Memory) is a volatile memory device for general use in all kinds of electronic products including a desktop PC and a portable phone. More particularly, the DRAM is a sort of RAM (Random Access Memory) in which memory cells capable of storing a value of 0 or 1 are structured in a form of an integrated circuit and a value of any memory cell can be read or changed. The RAM is distinguished into a DRAM, a SRAM (Static Random Access Memory), a PRAM, a MRAM and the like, according to the kind of the memory cells constructing the RAM.
The memory cells of the DRAM respectively include one capacitor and one transistor. The capacitor and the transistor are electric devices, in which the capacitor reserves a predetermined amount of charges and the transistor amplifies or switches electric signals. A process for reading contents stored in the memory cells of the DRAM is performed by selecting any one of the capacitors through the transistors and determining whether charges are reserved in the selected capacitor. Furthermore, a logical value of 0 or 1 can be recorded in the memory cells through a process of charging or discharging the charges in/from the capacitors.
The DRAM includes a plurality of banks which respectively have a plurality of cell arrays. The plurality of cell arrays respectively have plural memory cells. The plural memory cells are connected with corresponding word lines and corresponding bit lines so as to form a cell array.
When a high voltage is applied to the word line, electricity may flow between the capacitors and the bit lines by an operation of the transistors of the memory cell. At this time, if the charges are charged in the capacitors, it is possible to detect a fine difference of the voltage through the bit lines, thereby reading contents of the memory cell.
The DRAM has various problems caused by using the capacitors in the memory cell. The charges in the capacitor are gradually discharged in a time lapse. Therefore, when the time lapses, data stored in the memory cells are broken.
Since the charges accumulated in the capacitor of the memory cell gradually disappear according to passage of time, the capacitor constructing the memory cell in the DRAM should be periodically refreshed. That is, the charges accumulated in the memory cell must be maintained to the extent that they are normally amplified in a reading process. A time necessary for maintaining the amount of charges in the memory cell at a readable level is referred to data retention time. That is, the memory cell of the DRAM must be refreshed each data retention time.
Therefore, in the DRAM, the refreshing process for data retention is repeatedly performed to constantly amplify the charges accumulated in the capacitor of the memory cell so as to refresh the capacitor, even though contents stored in the memory cell are not read or written.
During an operation of software such as an operating system, an operation of moving contents stored in a main memory (this means a DRAM having a volatile property) to a certain location in the main memory is frequently generated. That is, data in a certain area are frequently moved to another area in the memory.